1. Field of the Invention
The present invention relates to a transient analysis device which conducts transient analysis processing of an analog/digital mixed circuit to high precision and at high speed.
2. Description of the Related Art
As techniques of simulating operation of a circuit by means of a computer, various proposals have been made for a technique of conducting transient analysis of an analog/digital mixed circuit in which both an analog circuit and a digital circuit exist. One conventional analog/digital mixed circuit transient analysis technique is disclosed, for example, in Japanese Patent Laying-Open (Kokai) No. Heisei 3-269673, entitled "Analog/Digital Mixed Circuit Simulation Method". The literature discloses an analysis method intended to reduce a time required for transient analysis. Another transient analysis method of a kind is recited in "C. W. Gear: The Spice Book" (written by Andrei Vladimirescu, John Wiley & Sons, Inc.) These literatures recite algorithms of the Variable-Timestep method by Gear. FIG. 7 is a flow chart showing a conventional analog/digital mixed circuit transient analysis method recited in these literatures. FIG. 8 is a simulation equivalent block diagram of a device which conducts the transient analysis shown in FIG. 7.
With reference to FIGS. 7 and 8, first, generate a circuit diagram of an analog/digital mixed circuit 61 as a target for transient analysis (Step 701). Concrete example of structure of the analog/digital mixed circuit 61 is shown in FIG. 12. A 16-bit analog/digital mixed circuit shown in FIG. 12 includes a digital circuit 73 for receiving input of an operation clock signal 101 to output switching signals 1201 to 1208, 24 switches provided corresponding to a reference voltage V.sub.REF, a ground potential and inputs of the switching signals 1201 to 1208, three OP amps. 70, 12 capacitors 71 and four resistors 72. The digital/analog mixed circuit then functions responsive to input of the operation clock signal 101 applied to the digital circuit 73 to output such an output signal 102 as shown in FIG. 14 through the OP amp. 70 in the last stage. FIG. 13 is a waveform diagram showing the operation clock signal 101 and the switching signals 1201 to 1208.
Next, convert the analog/digital mixed circuit 61 into a net list adapted to transient analysis simulation processing (Step 702), so that a simulator 62 conducts transient analysis using the generated net list (Step 703).
In the above-described conventional art, an analog circuit analysis algorithm for use in transient analysis is assumed to employ a variable analysis step method. The variable analysis step method represents an algorithm on which the amount of change in a potential of the analog/digital mixed circuit 61 output in response to the input of the operation clock signal 101 is traced to conduct analysis, for example, as shown in FIGS. 9 and 10, at a time step with a close pitch in time with respect to the operation causing the amount of potential change to have a sharp level-change (see an area J in FIG. 9 and an area N in FIG. 10) and conduct analysis at a time step with a loose pitch with respect to the operation causing the slow potential change (see an area K in FIG. 9 and an area O in FIG. 10).
According to the transient analysis method using this variable analysis method, conducting analysis with variable analysis steps closely set so as to generate an analysis point also for an operation part whose signal level change is slow in an analog circuit unit (see an area O in FIG. 10) results in having an increased number of analysis points for a signal of an operation part whose level change is sharp in the analog circuit unit (see an area N in FIG. 10) to increase an analysis time. On the contrary, when analysis are conducted with variable analysis steps loosely set in order to reduce an analysis time, while the number of analysis points for a signal of an operation part whose level change is sharp in the analog circuit unit (see an area J in FIG. 9) is reduced to reduce a transient analysis time, no analysis point is generated for a signal of an operation part whose level change is slow in the analog circuit unit (see an area K in FIG. 9), whereby a difference in a value of a simulator analysis signal from a value of a true output signal is increased to significantly decrease analysis precision.
As an example, when the analog/digital mixed circuit 61 as a target for analysis shown in FIG. 8 conducts FFT (fast Fourier transform) analysis of the 16-bit analog/digital mixed circuit shown in FIG. 12, conducting transient analysis to high precision requires an analysis point to be set also for a signal of a slow operation part in an analog unit (see the area O in FIG. 10). When an analysis point in question can not be obtained, transient analysis of the FFT in question for conducting analysis of a frequency and an amplitude value result in obtaining such a simulator analysis signal 105 as shown in FIG. 9 which has a value with a considerably large difference from that of the output signal 102 as a true output value. Conducting transient analysis demanded to have high analysis precision by the variable analysis step method therefore needs analysis steps to be closely set as illustrated in FIG. 10, which results in increasing the number of analysis points and an analysis time accordingly.
Another possible analysis algorithm for use in transient analysis is a fixed analysis step method. The fixed analysis step method represents an algorithm on which a potential of the output signal 102 of the analog circuit corresponding to an input of the operation clock signal 101 is analyzed at an analysis step T.sub.S with a fixed pitch as shown in FIG. 11. The fixed analysis step method has an advantage in that for the verification of a linear circuit element, analysis is possible in a relatively short transient analysis time.
In conventional transient analysis of an analog-digital mixed circuit, algorithms of the variable analysis step method or the fixed analysis step method are appropriately used to conduct transient analysis with respect to an analog/digital mixed circuit also including a non-linear circuit. In the simulation requiring analysis results with particularly high precision, when transient analyses are conducted according to a transient analysis method based on the fixed analysis step method, use of loose analysis steps T.sub.S prevents analysis results obtained by the simulation from converging and on the contrary, use of too close analysis steps T.sub.S enables analysis results to be converged but requires more analysis time as compared with that of a transient analysis method based on the variable analysis step method.
For conducting highly precise transient analysis of a complicated analog/digital mixed circuit according to a transient analysis method based on variable analysis steps, if variable analysis steps are closely set so as to generate an analysis point also at a slow operation part in an analog circuit, the number of analysis points at a sharp operation part in the analog circuit unit is increased to accordingly increase an analysis time.
On the contrary, loosely setting variable analysis steps in order to reduce an analysis time prevents generation of an analysis point at a slow operation part in the analog circuit unit, whereby an error in a value of a simulator analysis signal is increased to significantly degrade analysis precision.
In brief, a transient analysis method employing the fixed analysis step method has a shortcoming that improvement in analysis precision prevents analysis results of a non-linear element from converging, while a transient analysis method employing the variable analysis step method has a shortcoming that measures for improving analysis precision are inconsistent with measures for reducing an analysis time.
Moreover, with the above-described conventional transient analysis technologies, it is very difficult to designate such analysis steps as enable a minimum analysis time to be obtained within a range in which analysis precision necessary for an analog/digital mixed circuit to be analyzed is attained. It is therefore necessary to determine based on results obtained by actual execution of transient analysis whether required analysis precision is attained or not and when required precision is not attained, repetition of transient analysis with target precision further increased is essential, which results in further increasing a time required for analysis.